The present invention relates to an electrical filter circuit, and in particular, an electrical filter circuit which includes a bypass mode for bypassing a filtering portion of the circuit.
Filter circuits are well known in the art and are used in various applications. Filters are often used widely in telecommunications applications. For instance, wireless (e.g. cellular) communications equipment include various filters for performing the to different functions of the equipment. An example may include a reconstruction filter which follows a digital to analog (D/A) conversion device in either of the transmit or receive channels of the wireless equipment.
One well known filter circuit is the Salen and Key filter. The Salen and Key filter is an active filter which includes two basic elements: a filtering element and a amplification element. FIG. 1 shows a conventional second order low pass Salen and Key filter 10. The filter 10 includes an amplifier 20 for providing amplification, and an R-C network including two capacitors 30, 40 and two resistors 50, 55 for providing filtering. The filter 10 also includes resistors 60 and 65 which set the gain of the amplifier 20. The filter 10 includes an input terminal 12 and an output terminal 14. Signals which are presented at the input terminal 12 of the filter 10 are attenuated (and appear as such as output terminal 14) according to the passband of the low pass filter formed by capacitors 30, 40 and resistors 50, 55. Signals which are within the passband are amplified by amplifier 20 with a particular gain set by resistors 60 and 65.
Active filters such as Salen and Key filter 10 described above may be used for a variety of different reasons, including: (1) the filter function is required to multiple order, (2) large signal swing requirements, (3) linearity requirements, and (4) output drive requirements. It should be noted that the component values of the capacitors (e.g. capacitors 30, 40) of the filter 10 can be adjusted to more accurately tune the passband of the filter. This is typically accomplished by making one of the capacitors 30, 40 a variable capacitor, tuning that capacitor and a resistor (e.g. resistor 50 or 55) to a known time reference, and then tuning the other elements in accordance therewith.
Other examples of conventional active filter structures are the second order Rausch filter and the xe2x80x9cleakyxe2x80x9d integrator filter shown in FIGS. 2 and 3, respectively. The Rausch filter 100 includes an amplifier 120 for providing amplification, and an R-C network including two capacitors 130, 140 and three resistors 150, 155, 160 for providing filtering. The xe2x80x9cleakyxe2x80x9d integrator filter 200 shown in FIG. 3 includes an amplifier 220 and an R-C network including one capacitor 230 and two resistors 250, 260 for providing filtering. It will be noted that the filters shown in FIGS. 2 and 3 (e.g. Rausch and xe2x80x9cleakyxe2x80x9d integrator) have similar construction to the Salen and Key filter (shown in FIG. 1), but are slightly different as to the placement of the resistors and capacitors. However, all three conventional filters described above include capacitors which contribute to the filtering functions of the circuits.
Another conventional filter circuit 600 is shown in FIG. 4. FIG. 4 shows a full-differential third order Rausch filter. The filter 600 includes an amplifier 620 for amplification, and an R-C network including capacitors 630, 635, 640, and 645 and resistors 650-657 for providing filtering. The filter 600 differs from the other conventional filter circuits 10, 100, and 200 described above in that it includes balanced input 610, 611 and output 612, 613 terminals rather than unbalanced input and output terminals, and the amplifier 620 includes two output terminals, instead of just one.
In a conventional filter circuit applications, such as those described above, a mode may exist where the filtering function of the filter circuit is not desired, but the loading conditions on the output remain unchanged. Previous solutions to this problem required at least two buffer circuits, one with filtering elements and one without filtering elements, to accommodate both modes of operation. The requirement of two buffers requires additional area on either the silicon die or the circuit board on which the filter circuit is disposed.
Thus, there is currently a need for a combined filter and buffer circuit which includes a filter bypass mode.
The present invention is method and apparatus for providing an electrical circuit which includes a filter bypass mode. The method includes the steps of: providing at least one switched filtering element in a filtering circuit, and switching the filtering element out of the filter circuit in order to bypass the filter circuit.
The above and other advantages and features of the present invention will be better understood from the following detailed description of the preferred embodiments of the invention which is provided in connection with the accompanying drawings.